European Patent document EP-A-0 129 275 discloses a series of thin walled sheets which are joined together by locks which are formed with thick walls along the vertical free edges of the sheets. Each lock consists of a male end adapted to be fixed to a sheet and a female end adapted to be fixed to an adjacent sheet. The male and female ends interlock the sheets to create a watertight screen. The female end is provided with a longitudinally extending slot which extends along the length of its thick wall. Free edges of the slot are turned in toward one another such that the slot can receive a head portion of the male end of the lock. The head portion is retained in the slot by the free edges of the slot.
Accordingly, when connecting two adjacent sheets together it is necessary to provide a first sheet with a male locking portion and a second sheet with a female locking portion to enable the respective lock components to interlock. Thus, the sheets provided with the lock components must be oriented in the correct direction to ensure that male and female lock components are adjacent one another to enable the lock components to interlock to form the screen. Additionally, the production of a lock having a male and a female portion requires the use of two moulds to produce matched pairs of lock components. Further, when a supplier provides an end user with the matched pairs of lock components, equal quantities of both male and female lock components must be supplied. Consequently, two extruders are required to operate simultaneously if the producer wishes to avoid downtime caused by changing moulds on a single extruder.
An expansion seal is located in the slot of the female end to improve the sealing quality of the lock thereby providing a watertight joint. Usually, upon contact with moisture, an expansion seal will swell to a larger size to seal small gaps. However, it is normal practice for the expansion seal to be manually inserted into the slot on site. The situation may arise where a construction worker either neglects or purposely omits inserting the seal thereby reducing the integrity of the watertight seal between the locks. Moreover, a broken seal or two seals can sometimes be inserted in the slot such that the seal is not continuous along the length of the slot. This may also serve to reduce the quality of the seal.
Further, expansion seals include hydrophilling agents which are mixed with a formulation, the major constituent of which nitryl rubber. As the seal comes into contact with water and the seal expands, some of the hyrophilling agent is released into the water or liquid surrounding the seal. When the seal begins to dry, the seal shrinks and, because some of the hydrophilling agent has been lost to the water surrounding the seal, the next time the seal becomes wet, the seal is unable to expand to its originally expanded state. Thus, over time, the integrity of the seal in the interlocking joint is reduced thereby making the seal less effective.